Correlation between the luminosity and spin-period changes during outbursts of 12 Be binary pulsars observed by the MAXI/GSC and the Fermi/GBM

Abstract

To observationally study spin-period changes of accreting pulsars caused by the accretion torque, the present work analyzes X-ray light curves of 12 Be binary pulsars obtained by the MAXI/GSC all-sky survey and their pulse periods measured by the Fermi/GBM pulsar project, both covering more than 6 years from 2009 August to 2016 March. The 12 objects were selected because they are accompanied by clear optical identification, and accurate measurements of surface magnetic fields. The luminosity $L$ and the spin-frequency derivatives $\dot{\nu }$, measured during large outbursts with $L\gtrsim 1\times 10^{37}$ erg s${}^{-1}$, were found to approximately follow the theoretical relations in the accretion torque models, represented by $\dot{\nu }\propto L^{\alpha }$ ($\alpha \simeq 1$), and the coefficient of proportionality between $\dot{\nu }$ and $L^{\alpha }$, agrees, within a factor of $\sim 3$, with that proposed by Ghosh & Lamb (1979). In the course of the present study, the orbital elements of several sources were refined.

21 pages, 9 figures, accepted for publication in PASJ

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